The present invention relates generally to digital receivers which operate in an environment in which known co-channel interference is present, and, more particularly, to a digital television receiver, such as an HDTV receiver, which utilizes a rejection filter for cancellation of known co-channel interference, e.g., co-channel NTSC interference, and an equalizer for equalizing multipath channels without attempting to equalize the known co-channel interference.
The Federal Communications Commission (FCC) has recently approved an advanced television (ATV) standard which encompasses high definition television (HDTV) and standard definition television (SDTV) signals for terrestrial broadcasting. The HDTV signals will be encoded in accordance with the MPEG-2 coding protocol (as described in the ISO/IEC 13818 document), where "MPEG" is an acronyn for the "Moving Pictures Experts Group" which proposed this coding standard. The RF transmission scheme which will likely be used is the trellis-coded 8-VSB (Vestigial SideBand) system developed by Grand Alliance member Zenith Electronics. This system is described in detail in a publication entitled "VSB Transmission System: Technical Details", Feb. 18, 1994, the disclosure of which is incorporated herein by reference.
The FCC will require that ATV signals initially (for at least several years) be broadcast using currently unused analog NTSC television channels (sometimes referred to as "taboo" channels), since ATV broadcasting systems will, at least for this initial period, have to co-exist with conventional analog NTSC broadcasting systems. The resultant simultaneous broadcasting of digital ATV and analog NTSC television signals is oftentimes referred to as "simulcasting". A practical HDTV receiver must be capable of cancelling the resultant co-channel NTSC interference without excessively enhancing noise, in order to function properly. In this regard, a number of different solutions have been previously proposed, as summarized below.
An 8-VSB system developed by Zenith Electronics combats co-channel NTSC interference by using a comb filter in the HDTV receiver to introduce nulls in the digital spectrum at the frequency locations of the NTSC picture, color, and sound carriers. When co-channel NTSC interference is present at the HDTV receiver, the comb filter is treated as a partial response channel in cascade with the trellis decoder. A significant drawback of this approach to combating co-channel NTSC interference is that the performance of the comb filter, and thus, the overall performance of the HDTV receiver, is significantly degraded when co-channel NTSC interference and a high level of additive white Gaussian noise (AWGN) are present in the received signal. This is because the AWGN does not remain white after it is filtered by the comb filter, but gets "colored", meaning that the noise samples are not taken independently of each other. This, in turn, adversely affects the performance of the trellis decoder, which is optimized for performance in an AWGN channel. Another significant drawback of this approach to combating co-channel NTSC interference is that the comb filter must be switched out in the absence of co-channel NTSC interference, because it would otherwise excessively enhance noise.
U.S. Pat. No. 5,291,291, issued to Eilers, discloses an ATV system with reduced co-channel NTSC interference, in which the NTSC receiver subjective random noise sensitivity characteristic is utilized to shape the ATV transmitter power curve. A complementary filter is incorporated in the ATV receiver for compensating for the shaped ATV transmitter power curve. The shaped ATV power curve emphasizes signals at the frequencies where the NTSC subjective random noise sensitivity is low and deemphasizes signals at the frequencies where NTSC subjective sensitivity is high. Significant drawbacks of this approach to combating co-channel NTSC interference are that it requires modification of the ATV transmission system, and requires costly and complex modifications of the ATV receiver.
U.S. Pat. Nos. 5,452,015 and 5,512,957, both of which issued to Hulyalkar, and both of which are assigned to the assignee of the present invention, disclose an ATV system including an encoding/transmission system which includes a "bi-rate" control block (to select between 8-VSB and 4-VSB modulation) and respective 8-VSB and 4-VSB "trellis-precoding" blocks, and an ATV receiver having a decoder which is designed to process the subset-limited trellis pre-coded ATV signal which is transmitted by the transmitter. The disclosures of these two patents are incorporated herein by reference.
The Hulyalkar ATV receiver utilizes a co-channel NTSC inteference rejection filter and a decoder which processes co-channel NTSC interference in such a manner as to produce a residual interference spectrum which is as flat as possible at the output of the rejection filter. The filter exploits the fact that only the picture and the sound carriers need to be sufficiently attenuated and cancels co-channel NTSC interference with only a small degradation in performance when AWGN is present. Drawbacks of this approach to combating co-channel NTSC interference are that it requires the use of a "subset-limited trellis-coding" precoder in the transmitter of the ATV transmission system (thus changing the transmission stream) and a corresponding decoder and rejection filter in the receiver of the ATV transmission system.
U.S. Pat. No. 5,572,249, issued on Nov. 5, 1996, the disclosure of which is incorporated herein by reference, the inventor of which is the present inventor, and the assignee of which is the assignee of the present invention, discloses a filter which can be used for co-channel NTSC inteference cancellation in an ATV system, without introducing excessive noise enhancement. More particularly, this filter could be used in a precoder in the ATV transmitter and the same filter used as a co-channel NTSC interference rejection filter in the ATV receiver. Of course, this entails the same drawbacks discussed above in connection with the Hulyalkar patents.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a digital receiver which overcomes the above-described drawbacks of the presently available technology. More particularly, there presently exists a need in the art for a digital receiver, e.g., an HDTV or ATV receiver, which is provided with a co-channel interference rejection filter which enables the realization of an ATV system in which the rejection filter in the receiver can be used alone to cancel co-channel interference, without requiring a precoder in the transmitter and without requiring any change in the transmitted bitstream. The present invention fulfills this need in the art.